/*
 * Copyright (C) 2012 eXo Platform SAS.
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This software is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this software; if not, write to the Free
 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
 */
package org.jvss.hash;

/**
 * This class generates a cumulative CRC value upon the contents of multiple
 * buffers, allowing a CRC value to be generated for data or a file in piecemeal
 * fashion, a block at time.
 * 
 * For example, this code could be used to generate a CRC value for a file by
 * first instantiating a CRC32 object. The first few K of the file can then be
 * read into a buffer, and the contents of that buffer passed to
 * AccumulateBuffer(). This would then be repeated, continually reading more
 * data into the buffer and passing that data to AccumulateBuffer() until the
 * end of the file is reached. The final CRC value can then be obtained by
 * calling RetrieveCRC(). The CRC32 object can then be Reset() so that the
 * object can be reused to calculate the CRC value for other data.
 * 
 * Utility functions: CRC32() Instantiate a new CRC object, initializing it for
 * immediate use.
 * 
 * Reset() Reset the current incremental CRC value in preparation for
 * calculating a new CRC for a new group of data.
 * 
 * RetrieveCRC() Retrieve the current CRC value contained in the object.
 * 
 * AccumulateBuffer() Given a buffer of data, update the current CRC value
 * across all of the data contained in this buffer. This function will usually
 * be called multiple times for each block of data within the file or data
 * structure for which the CRC is being generated. NOTE: It is vital that data
 * which is passed to this function always be passed in the same order! If the
 * blocks of data are passed in a different order, then a different CRC value
 * will be generated.
 * 
 * Note: This code is based upon sample code written by Tomi Mikkonen
 * (tomitm@remedy.fi), including the CRC algorithm and the look-up table.
 */
public class VssCRC32
{
   static final int[] g_CrcTable = {0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F, 0xE963A535,
      0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91,
      0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7, 0x136C9856,
      0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E,
      0xD56041E4, 0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6,
      0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59, 0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116,
      0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924, 0x2F6F7C87,
      0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F,
      0x9FBFE4A5, 0xE8B8D433, 0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97,
      0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457,
      0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65, 0x4DB26158,
      0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC,
      0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA, 0xBE0B1010,
      0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F, 0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4,
      0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A, 0xEAD54739,
      0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D,
      0x0A00AE27, 0x7D079EB1, 0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 0x196C3671,
      0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5,
      0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B, 0xD80D2BDA,
      0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A,
      0x256FD2A0, 0x5268E236, 0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92,
      0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D, 0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A,
      0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B,
      0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B,
      0x6FB077E1, 0x18B74777, 0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69, 0x616BFFD3,
      0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB,
      0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9, 0xBDBDF21C,
      0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8,
      0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D};

   private int m_CRC;

   /**
    * 
    */
   public VssCRC32()
   {
      reset();
   }

   /**
    * Resets the CRC value so that this object can be reused to calculate CRC
    * values for other blocks of data.
    */
   public void reset()
   {
      m_CRC = 0xFFFFFFFF;
   }

   /**
    * @return Returns the current CRC value.
    */
   public int getM_CRC()
   {
      return m_CRC;
   }

   /**
    * Given a buffer of data, update the current CRC value across all of the
    * data contained in this buffer. This function will usually be called
    * multiple times for each block of data within the file or data structure
    * for which the CRC is being generated.
    * 
    * NOTE: It is vital that data which is passed to this function always be
    * passed in the same order! If the blocks of data are passed in a different
    * order, then a different CRC value will be generated.
    * 
    * @param buffer
    * @param bufferSize
    */
   public void accumulateBuffer(byte[] buffer, int bufferSize)
   {
      if (null == buffer)
      {
         return;
      }

      int kBits;
      int tCRC = m_CRC;
      for (int index = 0; index < bufferSize; ++index)
      {
         kBits = (tCRC ^ buffer[index]) & 0xFF;
         tCRC = tCRC >> 8 & 0xFFFFFF ^ g_CrcTable[kBits];
      }

      m_CRC = tCRC;
   }

   // Alternate version of the CRC-32 logic used for VSS.  It starts from zero
   // instead -1.
   //
   public static int VssCrc32(byte[] buffer, int offset, int bufferSize)
   {
      // Unlike other implementation, the VSS version starts accumulating from
      // 0x00000000 instead of 0xFFFFFFFF.
      int kBits;
      int tCRC = 0;
      for (int index = offset; index < bufferSize; ++index)
      {
         kBits = (tCRC ^ buffer[index]) & 0xFF;
         tCRC = tCRC >> 8 & 0xFFFFFF ^ g_CrcTable[kBits];
      }

      return tCRC;
   }

   public static short VssCrc16(byte[] buffer, int offset, int bufferSize)
   {
      // Unlike other implementation, the VSS version starts accumulating from
      // 0x00000000 instead of 0xFFFFFFFF.
      int kBits;
      int tCRC = 0;
      for (int index = offset; index < bufferSize; ++index)
      {
         kBits = (tCRC ^ buffer[index]) & 0xFF;
         tCRC = tCRC >> 8 & 0xFFFFFF ^ g_CrcTable[kBits];
      }

      // Fold the 32-bit CRC in on itself with XOR.
      return (short)(tCRC ^ tCRC >> 16);
   }
}
